remoteproc: k3: Refactor mailbox rx_callback functions into common driver

remoteproc: k3: Refactor mailbox rx_callback functions into common driver

The mailbox .rx_callback implementations in TI K3 R5, DSP and M4
remoteproc drivers handle inbound mailbox messages in the same way.
Introduce a common driver 'ti_k3_common.c' and refactor the
implementations into a common function 'k3_rproc_mbox_callback'() in it.

Signed-off-by: Beleswar Padhi <b-padhi@ti.com>
Tested-by: Judith Mendez <jm@ti.com>
Reviewed-by: Andrew Davis <afd@ti.com>
Link: https://lore.kernel.org/r/20250513054510.3439842-14-b-padhi@ti.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>